The invention relates to a bell made of opaque fused silica, preferably for use in the deposition of polycrystalline silicon and with an inside diameter of over 500 mm and a height of over 1300 mm.
From pamphlet Q-B 3/113 (September, 1979), pp. 8-9, of Heraeus Quarzschmelze GmbH there are known one-piece bells made of opaque fused silica (a fused product, obtained primarily from quartz sands and therefore of lower purity, and containing finely dispersed air bubbles that render it nontransparent). They are for the deposition of polycrystalline silicon and have inside diameters ranging from 500 to 900 mm and heights of up to 2200 mm.
In the deposition of polycrystalline silicon, the bells are heated at widely differing rates over their length due to the nature of the process; the two ends remain considerably cooler than the highly heated middle zone. Especially in the case of larger bells, this results in stresses and fractures due to hindered thermal expansion.